An LED unit is able to convert the electrical power directly into the light power. Due to its swift response, a relatively small size, low power consumption, low working voltage to drive the same, long service life (from 100 thousand hours to 10 million hours), firm stability, tolerable to high vibration and low manufacture cost, the LED units are widely used in the display devices, or used as an optical fiber light source applied to short-distance communication. The late LCD devices are implemented with a backlight module that uses the LED units therein. Thus, a research leading to improvement of the LED unit becomes an important object for the manufacturers.
The LED unit includes a die and two electric wires and is said to be compatible with the Lambert's cosine law. Thus, the LED unit is sometimes called a Lambertian source. Its specific feature is that the light emitting strength of the LED unit is directly proportional to a cosine of the viewing angle. The brightness values thereof and the viewing angle have no interrelationship. In order to control the brightness in different directions, a lens is disposed above the die within the packaged LED so as to guide the light rays emitted from the die. FIG. 1A shows a conventional lens 10 employed in the prior LED unit. The conventional lens 10 has a dome-shaped top portion, providing a viewing angle ranging 0° to 90°.
FIG. 1B illustrates a candela distribution representing brightness values varying with the viewing angles when the light rays pass through the conventional lens 10. When the viewing angle is 0°, the lens 10 provides the maximum value of brightness 90×103 nits, which decreases as the viewing angle is increased. When the viewing angle is increased to 90°, the brightness value is 0 nits. Therefore, it is observable that in the conventional lens 10, the light rays generally collect to the viewing angle 0° after passing through the lens 10. FIG. 1C illustrates a graph of a simulation of a backlight module, within which the LED unit implementing the conventional lens 10 is provided. The vertical axis indicates the brightness values while the horizontal axis indicates the distance. By observing the fluctuation of the curve, it can be noted that there is uneven brightness in the different regions.
In order to provide uniform light distribution in the backlight module, a direct-type backlight module with LED units from Lumileds Co. is preferred to when compared with the other backlight modules.
FIG. 2A is an exploded view of the direct-type backlight module 20 with LED units. The backlight module 20 includes a receptacle body 21 (generally a metal casing), a plurality of LED units 22 mounted on a bottom plate 211 of the receptacle body 21 in array in order to form two light bars. A diffusing plate 23 is disposed above the receptacle body 21 to diffuse the light rays emitted from the LED units 22.
FIG. 2B shows a packaged LED unit 22 employed in the backlight module of FIG. 2A, and includes an outer casing 221, a lens 222 and two fastening pieces 2211 projecting outwardly from the casing 221 in order to fix the packaged LED unit 22 to the bottom plate 211 of the receptacle body 21. A die (not visible) is enclosed within the outer casing 221. The lens 222 is fixed on top of the outer casing 221 to guide the light rays emitted from the die in ±90°.
FIG. 2C is an LED lens 222 designed and produced by the Lummiled Co., and has an inverted cone-shaped top portion formed with concave inner surface to reflect the light rays therefrom. Thus, the light rays emitted from within the outer casing 221 are reflected and diffused from the concave inner surface of the top portion of the LED lens 222 in ±90° FIG. 2D is a graph representing the brightness values with respect to the viewing angles of the LED unit 22 produced by the Lummiled Co. As shown, the light brightness is centered between the 80±20°. In other words, the light rays are barely emitted from the right top of the LED lens 222. FIG. 2E illustrates a graph of a simulation of the backlight module provided with the LED lens 222. The vertical axis indicates the brightness values (nits) while the horizontal axis indicates the distance. By observing the fluctuation of the curve, it can be noted that there is an appropriate brightness uniform in the different regions when the LED lens 222 of the Lummiled Co. is utilized in the backlight module.
From the aforesaid paragraphs, it can be noted that utilization of the conventional lens 10 provides non-uniform brightness in the different viewing angles while the LED lens 222 of the Lummiled Co. provides an appropriate brightness uniform. However, the cost for manufacturing the Lummiled's LED lens 222 is relatively high. It is considerably hard to design the inverted cone-shaped top portion of the Lummiled's LED lens 222 since its structure is complicated.